The present invention relates to an apparatus and a method for playback and a recording medium, and particularly to an apparatus and a method for playback and a recording medium suitable for use in preventing a malfunction of a servo of an optical pickup resulting from a flaw or the like on a disk medium, for example.
FIG. 1 shows a configuration of a conventional optical disk playback apparatus for reproducing data recorded on an optical disk.
A spindle motor 2 in the conventional optical disk playback apparatus drives and rotates an optical disk 1. An optical pickup 3 irradiates the optical disk 1 with laser light, generates a signal corresponding to the reflected light, and then outputs the signal to an RF signal generating unit 4, an FE signal generating unit 5, and a TE signal generating unit 6. Also, the optical pickup 3 adjusts focus servo operation in response to a focus drive signal from a focus driver 10, and adjusts tracking servo operation in response to a tracking drive signal from a tracking driver 12.
The RF signal generating unit 4 generates an RF signal on the basis of the signal from the optical pickup 3, and then outputs the RF signal to a binarizing unit 7 and a defect detecting unit 8. The FE signal generating unit 5 generates a focus error signal (hereinafter described as an FE signal) on the basis of the signal from the optical pickup 3, and then outputs the FE signal to a focus servo control unit 9. The TE signal generating unit 6 generates a tracking error signal (hereinafter described as a TE signal) on the basis of the signal from the optical pickup 3, and then outputs the TE signal to a tracking servo control unit 11.
The binarizing unit 7 generates a data signal by binarizing the RF signal from the RF signal generating unit 4 into 0 or 1. The defect detecting unit 8 detects a loss of a signal (defect) resulting from a flaw, a stain or the like present on the optical disk 1 on the basis of the RF signal from the RF signal generating unit 4, generates a defect signal indicating a period of a defect detected, and then outputs the defect signal to the focus servo control unit 9 and the tracking servo control unit 11.
A method for detecting a defect is, for example, to set level of the defect signal during a period when level of the RF signal is lower than a predetermined threshold value as High, and to set the defect signal during a period when the level of the RF signal is higher than the predetermined threshold value as Low. Specifically, when the RF signal has a level as shown in FIG. 2A, the defect signal has a corresponding level as shown in FIG. 2B.
During a normal period (period when the level of the defect signal from the defect detecting unit 8 is Low), the focus servo control unit 9 generates a focus drive control signal corresponding to the FE signal from the FE signal generating unit 5, and then outputs the focus drive control signal to the focus driver 10. When the level of the defect signal is High, the focus servo control unit 9 holds level of the focus drive control signal at a predetermined reference value or a value of the focus drive control signal immediately before the level of the defect signal becomes High, as shown in FIG. 2D, and then outputs the level of the focus drive control signal to the focus driver 10.
The focus driver 10 generates a focus drive signal corresponding to the focus drive control signal from the focus servo control unit 9, and then outputs the focus drive signal to the optical pickup 3.
During a normal period (period when the level of the defect signal from the defect detecting unit 8 is Low), the tracking servo control unit 11 generates a tracking drive control signal on the basis of the TE signal from the TE signal generating unit 6, and then outputs the tracking drive control signal to the tracking driver 12. When the level of the defect signal is High, the tracking servo control unit 11 holds level of the tracking drive control signal at a predetermined reference value or a value of the tracking drive control signal immediately before the level of the defect signal becomes High, as shown in FIG. 2F, and then outputs the level of the tracking drive control signal to the tracking driver 12.
The tracking driver 12 generates a tracking drive signal corresponding to the tracking drive control signal from the tracking servo control unit 11, and then outputs the tracking drive signal to the optical pickup 3.
In some cases, the conventional optical disk playback apparatus supplies the defect signal generated by the defect detecting unit 8 to the FE signal generating unit 5 and the TE signal generating unit 6.
In such cases, during a normal period (period when the level of the defect signal from the defect detecting unit 8 is Low), the FE signal generating unit 5 supplied with the defect signal generates the FE signal on the basis of the signal from the optical pickup 3, and then outputs the FE signal to the focus servo control unit 9. When the level of the defect signal is High, the FE signal generating unit 5 holds level of the FE signal at a predetermined reference value or a value of the FE signal immediately before the level of the defect signal becomes High, as shown in FIG. 2C, and then outputs the level of the FE signal to the focus servo control unit 9. The focus servo control unit 9 outputs a focus drive signal as shown in FIG. 2D corresponding to the FE signal whose level is being held constant.
During a normal period (period when the level of the defect signal from the defect detecting unit 8 is Low), the TE signal generating unit 6 supplied with the defect signal generates the TE signal on the basis of the signal from the optical pickup 3, and then outputs the TE signal to the tracking servo control unit 11. When the level of the defect signal is High, the TE signal generating unit 6 holds level of the TE signal at a predetermined reference value or a value of the TE signal immediately before the level of the defect signal becomes High, as shown in FIG. 2E, and then outputs the level of the TE signal to the tracking servo control unit 11. The tracking servo control unit 11 outputs a tracking drive signal as shown in FIG. 2F corresponding to the TE signal having a fixed value.
Thus, even if normal reflected light is not obtained because of the presence of a flaw or the like on the optical disk 1, the conventional optical disk playback apparatus thus formed holds the level of the focus drive signal and the tracking drive signal while the level of the defect signal is High. Therefore, a malfunction of a focus servo and a tracking servo of the optical pickup 3 is prevented.
However, in the case of a long High-level period of the defect signal or depending on an error in the output of the held focus drive signal and tracking drive signal, the optical pickup 3 is displaced substantially from an original servo control position when the level of the defect signal returns to Low.
Japanese Patent Laid-Open No. Sho 59-203276, for example, discloses a method for preventing such displacement which, directing attention to characteristics of the TE signal such as periodicity, generates a pseudo error signal approximating an original TE signal by calculation using a tracking error and a track period, and uses the calculated pseudo error signal instead of the original TE signal while the level of the defect signal is High.
Also, Japanese Patent Laid-Open No. Sho 64-39638 discloses a method which changes focus offset adjusting voltage to a predetermined value to thereby prevent displacement of a focus coil while the level of the defect signal is High, and thus minimizes the error when the level of the defect signal returns to Low.
The conventional techniques described above are all principally aimed to reduce a range of the control error while the level of the defect signal is High and after the level of the defect signal returns to Low, and give no consideration to reduction of a period from the return to Low of the level of the defect signal to the return to a normal control state of the servo.
In general, as the High-level period of the defect signal becomes longer, the control error tends to be increased. Hence, in case where the control error is large if not departing from a normal servo control range when the level of the defect signal returns to Low, for example, as shown in FIGS. 3A, 3B, 3C, 3D, and 3E, the servo continues to be unstable and requires considerable time before the servo returns to a normal control state.
In order to return the servo quickly to the normal control state, there is known a method which maintains a servo loop gain at a higher-than-normal level immediately after a track jump or the like. However, when the servo loop gain is high, the servo is generally too sensitive to a defect such as a flaw on the optical disk. Therefore, the method of increasing the servo loop gain cannot be used to improve defect passage characteristics.